1. Field of the Invention
The present invention relates to a thin-film magnetic head comprising a magnetoresistive element, and to a head gimbal assembly and a hard disk drive each incorporating the thin-film magnetic head.
2. Description of the Related Art
Performance improvements in thin-film magnetic heads have been sought as areal recording density of hard disk drives has increased. A widely used type of thin-film magnetic head is a composite thin-film magnetic head that has a layered structure in which a write (recording) head having an induction-type electromagnetic transducer for writing and a read (reproducing) head having a magnetoresistive (MR) element for reading are stacked on a substrate.
MR elements include: anisotropic magnetoresistive (AMR) elements utilizing an anisotropic magnetoresistive effect; giant magnetoresistive (GMR) elements utilizing a giant magnetoresistive effect; and tunnel magnetoresistive (TMR) elements utilizing a tunnel magnetoresistive effect.
It is required that the characteristics of a read head include high sensitivity and high output capability. GMR heads incorporating spin-valve GMR elements have been mass-produced as read heads that satisfy such requirements.
A typical spin-valve GMR element incorporates: a nonmagnetic conductive layer having two surfaces facing toward opposite directions; a free layer disposed adjacent to one of the surfaces of the nonmagnetic conductive layer; a pinned layer disposed adjacent to the other of the surfaces of the nonmagnetic conductive layer; and an antiferromagnetic layer disposed adjacent to one of the surfaces of the pinned layer farther from the nonmagnetic conductive layer. The free layer is a layer in which the direction of magnetization changes in response to a signal magnetic field. The pinned layer is a ferromagnetic layer in which the direction of magnetization is fixed. The antiferromagnetic layer is a layer that fixes the direction of magnetization in the pinned layer by means of exchange coupling with the pinned layer. Such a GMR element is disclosed in the Published Unexamined Japanese Patent Application Heisei 11-97763 (1999), for example.
In a GMR head, typically, the GMR element is located between two shield layers disposed on top and bottom thereof. An insulating film is provided between the GMR element and each of the shield layers. Bias field applying layers are disposed on both sides of the GMR element that are opposed to each other in the direction of the track width. The bias field applying layers apply a bias magnetic field to the free layer. Such a GMR head is disclosed in the Published Unexamined Japanese Patent Application 2002-100011, for example.
The bias magnetic field generated by the bias field applying layers directs the magnetization in the free layer to the direction of track width while no signal magnetic field sent from the recording medium is applied to the free layer. The magnetization in the pinned layer is fixed to the direction orthogonal to a medium facing surface of the head that faces toward the recording medium. Consequently, an angle of 90 degrees is maintained between the direction of magnetization in the pinned layer and the direction of magnetization in the free layer while no signal magnetic field sent from the recording medium is applied to the free layer. If a signal magnetic field in the direction orthogonal to the medium facing surface is sent from the recording medium and applied to the GMR head, the direction of magnetization in the free layer is changed, and the angle between the direction of magnetization in the pinned layer and the direction of magnetization in the free layer is thereby changed. The electrical resistance of the GMR element is changed by this angle. Therefore, it is possible to read data stored on the medium by detecting the change in electrical resistance of the GMR element.
Many of GMR heads have a structure in which a top shield layer is adjacent to an end of the GMR element opposite to the medium facing surface, an insulating film being provided between the top shield layer and the end of the GMR element. The top shield layer is made of a material such as NiFe (80 weight % Ni and 20 weight % Fe). This NiFe (80 weight % Ni and 20 weight % Fe) has a linear thermal expansion coefficient of about 12×10−6/° C. at a temperature of 30° C. When a thin-film magnetic head incorporating the GMR head is actually operated, the temperature of the GMR head goes up to a hundred and several tens of degrees centigrade. As a result, when the thin-film magnetic head is actually operated, the top shield layer expands, and a force pressing toward the medium facing surface is thereby applied to the GMR element. Typically, there is hardly a case in which the magnetic material used for the free layer of the GMR element has a magnetostriction constant of zero, but the material has a magnetostriction constant of a specific positive or negative value. Therefore, a magnetic anisotropy is created in the free layer by the inverse magnetostrictive effect when the force pressing toward the medium facing surface is applied to the GMR element by the expansion of the top shield layer when the thin-film magnetic head is actually operated, as described above. The magnetic anisotropy created in the free layer functions such that the magnetization in the free layer is directed to the direction of track width or the direction orthogonal to the medium facing surface, depending on whether the magnetostriction constant of the magnetic material of the free layer is of a positive value or a negative value.
The asymmetry of the output of the head changes when the magnetic anisotropy resulting from the inverse magnetostrictive effect is created in the free layer as described above. The asymmetry of the output of the head means the asymmetry between two types of output waveforms of the head: one of the two types is a waveform obtained when a magnetic field of +H is applied to the head in the direction orthogonal to the medium facing surface; and the other of the two types is a waveform obtained when a magnetic field of −H is applied to the head in the direction orthogonal to the medium facing surface. To be specific, the asymmetry of the output of the head is expressed by the equation below where the amounts of change in resistance obtained when magnetic fields of +H and −H are applied to the head are indicated by ΔR(+H) and ΔR(−H), respectively.R(+H)+ΔR(−H)}(%)
Even though a number of thin-film magnetic heads having the same specifications are manufactured, there are variations among the heads in the force applied to each GMR element by the expansion of the top shield layer during actual use. Therefore, conventional heads have a problem that variations in asymmetry of the outputs of the heads during actual use increase, because of the expansion of the top shield layer.
As disclosed in the Published Unexamined Japanese Patent Application Heisei 11-97763, it is known that tensile stress in the direction orthogonal to the medium facing surface exists as internal stress in the GMR element in the GMR head. This publication discloses a technique in which the value of magnetostriction constant of the free layer is made −2×10−6 to 0 for preventing a deterioration of the asymmetry resulting from the tensile stress.
However, this technique is not capable of preventing an increase in variations in asymmetry resulting from the expansion of the top shield layer as described above when the value of the magnetostriction constant of the free layer is smaller than zero. In addition, the technique has a problem that limitation is imposed on selection of the magnetic material used for the free layer when the value of the magnetostriction constant of the free layer is zero.
The Published Unexamined Japanese Patent Application 2002-100011 discloses a technique in which a rear insulating film is provided to be adjacent to an end of the MR element opposite to the medium facing surface, and the rear insulating film is made of an insulating material having a thermal conductivity higher than that of the insulating film provided between the MR element and each of the shield layers. According to this technique, it is possible to suppress a rise in temperature of the MR element. However, the technique has the following problem since the rear insulating film is disposed on the side of the MR element opposite to the MR element, the rear insulating film being made of a hard inorganic insulating material and having a large volume. When the medium facing surface is polished during the manufacturing process of the thin-film magnetic head, stress is applied to the MR element. If the rear insulating film that is hard and has a large volume is disposed on the side of the MR element opposite to the medium facing surface, the stress applied to the MR element during polishing of the medium facing surface increases, and the direction of magnetization in the pinned layer may be thereby changed.
The foregoing problems are not limited to the case in which the MR element is a spin-valve GMR element but applicable to thin-film magnetic heads in general incorporating various types of MR elements.